Sorry bruv, most of the websites I used to buy blowgun gear from either dont exist anymore, or dont carry much in the way of .62 cal gear.
I see several DIY approaches to a good mouthpiece, but nowhere to just buy one. The type of mouthpiece you show is very, very common for .40 and .50 cals, but Ive only ever seen them a couple of times for a 62.
Kinda makes me wish I had a 3D printer cuz there's all kinds of gadgets could be dreamt up and built with one of those things!!
Good luck, and if ya find a place, lemme know, cuz my 3 favorite sites are now basically dead (for .62 cal, .40's and .50's a plenty, but they just aren't my jam after using big bores!)